Al-Cu　alloys　show　great　potential　for　aerospace　applications　because　of　their　remarkable　advantages,　and　wire　arc　additive　manufacturing　(WAAM)　provides　a　way　to　print　large-scale　metal　structures.　However,　the　high　crack　susceptibility　is　a　limiting　factor　for　WAAM　of　Al-Cu　alloy　parts.　In　this　study,　the　primary　mechanism　of　crack　formation,　including　the　grain　structure,　Cu　segregation,　and　eutectic　structure　transition,　of　an　as　-deposited　WAAM　Al-Cu　alloy　were　investigated.　The　results　showed　that　macrocracks　with　lengths　of　approxi-mately　6-9　cm　and　depths　of　5.6-7　mm　occurred　in　the　bottom　of　the　part.　The　cracks　in　the　WAAM　part　of　the　Al-Cu　alloy　were　dominated　by　liquation　cracks,　which　preferentially　formed　at　high-angle　grain　boundaries　(GBs)　with　GB　angles　in　the　range　30　degrees　-60　degrees.　Moreover,　the　discontinuous　normal　eutectics　transform　to　continuous-strip　divorced　eutectics　in　the　GBs.　This　is　clear　evidence　for　the　temperature　of　the　GB　liquid　in　the　partially　melting　zone　exceeding　the　eutectic　temperature　(548　degrees　C)　during　the　subsequent　re-heating　process.　An　almost　constant　amount　of　solute　segregation　was　calculated　by　Scheil　solidification　models.　The　microstructure　showed　the　fine　to　coarse　grain　transformation　in　a　deposited　layer,　resulting　in　lower　GB　density　toward　the　inner-layer　zone.　Thus,　the　conditions　that　are　needed　for　liquation　cracking　are　low　GB　density　to　generate　a　wide　eutectic　liquid　film　and　high　intergranular　strain　in　the　partially　melting　zone.　This　means　to　liquation　cracking　is　more　prefer　to　nucleated　in　comparison　with　solidification　cracking.